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Class XI
Chapter 16 – Digestion and Absorption
Biology
Question 1:
Choose the correct answer among the following:
(a) Gastric juice contains
(i) pepsin, lipase and rennin
(ii) trypsin lipase and rennin
(iii) trypsin, pepsin and lipase
(iv) trypsin, pepsin and renin
(b) Succus entericus is the name given to
(i) a junction between ileum and large intestine
(ii) intestinal juice
(iii) swelling in the gut
(iv) appendix
Answer
Answer (a): (i) Pepsin, lipase, and rennin
Gastric juice contains pepsin, lipase, and rennin. Pepsin is secreted in an inactive
form as pepsinogen, which is activated by HCl. Pepsin digests proteins into peptones.
Lipase breaks down fats into fatty acids. Rennin is a photolytic enzyme present in the
gastric juice. It helps in the coagulation of milk.
Answer (b): (ii) Intestinal juice
Succus entericus is another name for intestinal juice. It is secreted by the intestinal
gland. Intestinal juice contains a variety of enzymes such as maltase, lipases,
nucleosidases, dipeptidases, etc.
Question 2:
Match column I with column II
Column I
Column II
(a)
Bilirubin and biliverdin
(i)
Parotid
(b)
Hydrolysis of starch
(ii)
Bile
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Class XI
Chapter 16 – Digestion and Absorption
(c)
Digestion of fat
(iii)
Lipases
(d)
Salivary gland
(iv)
Amylases
Answer
Column I
Column II
(a)
Bilirubin and biliverdin
(ii)
Bile
(b)
Hydrolysis of starch
(iv)
Amylases
(c)
Digestion of fat
(iii)
Lipases
(d)
Salivary gland
(i)
Parotid
Question 3:
Answer briefly:
(a) Why are villi present in the intestine and not in the stomach?
(b) How does pepsinogen change into its active form?
(c) What are the basic layers of the wall of alimentary canal?
(d) How does bile help in the digestion of fats?
Answer
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Biology
Class XI
Chapter 16 – Digestion and Absorption
Biology
(a) The mucosal wall of the small intestine forms millions of tiny finger-like
projections known as villi. These villi increase the surface area for more efficient food
absorption.
Within these villi, there are numerous blood vessels that absorb the digested
products of proteins and carbohydrates, carrying them to the blood stream. The villi
also contain lymph vessels for absorbing the products of fat-digestion. From the
blood stream, the absorbed food is finally delivered to each and every cell of the
body.
The mucosal walls of the stomach form irregular folds known as rugae. These help
increase the surface area to volume ratio of the expanding stomach.
(b) Pepsinogen is a precursor of pepsin stored in the stomach walls. It is converted
into pepsin by hydrochloric acid. Pepsin is the activated in the form of pepsinogen.
Pepsinogen
Pepsin + Inactive peptide
(Inactive) (Active)
(c) The walls of the alimentary canal are made up of four layers. These are as
follows:
(i) Serosa is the outermost layer of the human alimentary canal. It is made up of a
thin layer of secretory epithelial cells, with some connective tissues underneath.
(ii) Muscularis is a thin layer of smooth muscles arranged into an outer longitudinal
layer and an inner circular layer.
(iii) Sub-mucosa is a layer of loose connective tissues, containing nerves, blood,
and lymph vessels. It supports the mucosa.
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Class XI
iv.
Chapter 16 – Digestion and Absorption
Biology
Mucosa is the innermost lining of the lumen of the alimentary canal. It is
mainly involved in absorption and secretion.
(d) Bile is a digestive juice secreted by the liver and stored in the gall bladder. Bile
juice has bile salts such as bilirubin and biliverdin. These break down large fat
globules into smaller globules so that the pancreatic enzymes can easily act on them.
This process is known as emulsification of fats. Bile juice also makes the medium
alkaline and activates lipase.
Question 4:
State the role of pancreatic juice in digestion of proteins.
Answer
Pancreatic juice contains a variety of inactive enzymes such as trypsinogen,
chymotrypsinogen, and carboxypeptidases. These enzymes play an important role in
the digestion of proteins.
Physiology of protein-digestion
The enzyme enterokinase is secreted by the intestinal mucosa. It activates
trypsinogen into trypsin.
Trypsinogen
Trypsin
then
Trypsin + Inactive peptide
activates
the
other
enzymes
of
pancreatic
juice
such
as
chymotrypsinogen and carboxypeptidase.
Chymotrypsinogen is a milk-coagulating enzyme that converts proteins into peptides.
Chymotrypsinogen
Chymotrypsin
(Inactive)
Proteins
(Active)
Peptides
Carboxypeptidase acts on the carboxyl end of the peptide chain and helps release
the last amino acids. Hence, it helps in the digestion of proteins.
Peptides
Smaller peptide chain + Amino acids
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Class XI
Chapter 16 – Digestion and Absorption
Biology
Thus, in short, we can say that the partially-hydrolysed proteins present in the
chyme are acted upon by various proteolytic enzymes of the pancreatic juice for their
complete digestion.
Dipeptides and proteases
Proteins, peptones
Question 5:
Describe the process of digestion of protein in stomach.
Answer
The digestion of proteins begins in the stomach and is completed in the small
intestine. The digestive juice secreted in the gastric glands present on the stomach
walls is called gastric juice. The food that enters the stomach becomes acidic on
mixing with this gastric juice.
The main components of gastric juice are hydrochloric acid, pepsinogen, mucus, and
rennin. Hydrochloric acid dissolves the bits of food and creates an acidic medium so
that pepsinogen is converted into pepsin. Pepsin is a protein- digesting enzyme. It is
secreted in its inactive form called pepsinogen, which then gets activated by
hydrochloric acid. The activated pepsin then converts proteins into proteases and
peptides.
Proteins
Proteases + Peptides
Rennin is a proteolytic enzyme, released in an inactive form called prorennin. Rennin
plays an important role in the coagulation of milk.
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Class XI
Chapter 16 – Digestion and Absorption
Biology
Question 6:
Given the dental formula of human beings
Answer
The dental formula expresses the arrangement of teeth in each half of the upper jaw
and the lower jaw. The entire formula is multiplied by two to express the total
number of teeth.
The dental formula for milk teeth in humans is:
Each half of the upper jaw and the lower jaw has 2 incisors, 1 canine, and 2 molars.
Premolars are absent in milk teeth.
The dental formula for permanent teeth in humans is:
Each half of the upper jaw and the lower jaw has 2 incisors, 1 canine, 2 premolars,
and 3 molars. An adult human has 32 permanent teeth.
Question 7:
Bile juice contains no digestive enzymes, yet it is important for digestion. Why?
Answer
Bile is a digestive juice secreted by the liver. Although it does not contain any
digestive enzymes, it plays an important role in the digestion of fats. Bile juice has
bile salts such as bilirubin and biliverdin. These break down large fat globules into
smaller globules so that the pancreatic enzymes can easily act on them. This process
is known as emulsification of fats. Bile juice also makes the medium alkaline and
activates lipase.
Question 8:
Describe the digestive role of chymotrypsin. What two other digestive enzymes of
the same category are secreted by its source gland?
Answer
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Class XI
Chapter 16 – Digestion and Absorption
Biology
The enzyme trypsin (present in the pancreatic juice) activates the inactive enzyme
chymotrypsinogen into chymotrypsin.
Chymotrypsinogen
Chymotrypsin
(Inactive)
(Active)
The activated chymotrypsin plays an important role in the further breakdown of the
partially-hydrolysed proteins.
Proteins
The
Peptides
other
digestive
enzymes
of
the
same
category
are
trypsinogen
and
carboxypeptidase. These are secreted by the same source-gland, pancreas.
Trypsinogen is present in an inactive form in the pancreatic juice. The enzyme
enterokinase – secreted by the intestinal mucosa – activates trypsinogen into
trypsin.
Trypsinogen
Trypsin + Inactive peptide
The activated trypsin then further hydrolyses the remaining trypsinogen and
activates
other
pancreatic
enzymes
such
as
chymotrypsinogen
and
carboxypeptidase. Trypsin also helps in breaking down proteins into peptides.
Proteins
Peptides
Carboxypeptidases act on the carboxyl end of the peptide chain and help in releasing
the last amino acids.
Peptides
Small peptide chain + Amino acids
Question 9:
How are polysaccharides and disaccharides digested?
Answer
The digestion of carbohydrates takes place in the mouth and the small intestine
region of the alimentary canal. The enzymes that act on carbohydrates are
collectively known as carbohydrases.
Digestion in the mouth:
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Class XI
Chapter 16 – Digestion and Absorption
Biology
As food enters the mouth, it gets mixed with saliva. Saliva – secreted by the salivary
glands – contains a digestive enzyme called salivary amylase. This enzyme breaks
down starch into sugar at pH 6.8.
Starch
Maltose + Isomaltose + Limit dextrins
Salivary amylase continues to act in the oesophagus, but its action stops in the
stomach as the contents become acidic. Hence, carbohydrate-digestion stops in the
stomach.
Digestion in the small intestine:
Carbohydrate-digestion is resumed in the small intestine. Here, the food gets mixed
with the pancreatic juice and the intestinal juice. Pancreatic juice contains the
pancreatic amylase that hydrolyses the polysaccharides into disaccharides.
Starch
Disaccharides
(Polysaccharides)
Similarly, the intestinal juice contains a variety of enzymes (disaccharidases such as
maltase, lactase, sucrase, etc.). These disaccharidases help in the digestion of
disaccharides. The digestion of carbohydrates is completed in the small intestine.
Maltose
2Glucose
Lactose
Glucose + Galactose
Sucrose
Glucose + Fructose
Question 10:
What would happen if HCl were not secreted in the stomach?
Answer
Hydrochloric acid is secreted by the glands present on the stomach walls. It dissolves
bits of food and creates an acidic medium. The acidic medium allows pepsinogen to
be converted into pepsin. Pepsin plays an important role in the digestion of proteins.
Therefore, if HCl were not secreted in the stomach, then pepsin would not be
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Class XI
Chapter 16 – Digestion and Absorption
Biology
activated. This would affect protein digestion. A pH of about 1.8 is necessary for
proteins to be digested. This pH is achieved by HCl.
Question 11:
How does butter in your food gets digested and absorbed in the body?
Answer
Digestion of fats:
Butter is a fat product and gets digested in the small intestine. The bile juice
secreted by the liver contains bile salts that break down large fat globules into
smaller globules, so as to increase their surface area for the action of lipase. This
process is referred to as emulsification of fats.
After this, the pancreatic lipase present in the pancreatic juice and the intestinal
lipase present in the intestinal juice hydrolyse the fat molecules into triglycerides,
diglycerides, monoglycerides, and ultimately into glycerol.
Fats
Triglycerides + Diglycerides
Diglycerides and monoglycerides
Fatty acids + Glycerol
Absorption of fats:
Fat absorption is an active process. During fat digestion, fats are hydrolysed into
fatty acids and glycerol. However, since these are water insoluble, they cannot be
directly absorbed by the blood. Hence, they are first incorporated into small droplets
called micelles and then transported into the villi of the intestinal mucosa.
They are then reformed into small microscopic particles called chylomicrons, which
are small, protein-coated fat globules. These chylomicrons are transported to the
lymph vessels in the villi. From the lymph vessels, the absorbed food is finally
released into the blood stream and from the blood stream, to each and every cell of
the body.
Question 12:
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Class XI
Chapter 16 – Digestion and Absorption
Biology
Discuss the main steps in the digestion of proteins as the food passes through
different parts of the alimentary canal.
Answer
The digestion of proteins begins in the stomach and is completed in the small
intestine. The enzymes that act on proteins are known as proteases.
Digestion in the stomach:
The digestive juice secreted in the gastric glands present on the stomach walls is
called gastric juice. The main components of gastric juice are HCl, pepsinogen, and
rennin. The food that enters the stomach becomes acidic on mixing with this gastric
juice.
The acidic medium converts inactive pepsinogen into active pepsin. The active pepsin
then converts proteins into proteases and peptides.
Proteins
Proteases + Peptides
The enzyme rennin plays an important role in the coagulation of milk.
Digestion in the small intestine:
The food from the stomach is acted upon by three enzymes present in the small
intestine – pancreatic juice, intestinal juice (known as succus entericus), and bile
juice.
Action of pancreatic juice
Pancreatic juice contains a variety of inactive enzymes such as trypsinogen,
chymotrypsinogen,
and
carboxypeptidases.
The
enzymes
are
present
in
an
inactivated state. The enzyme enterokinase secreted by the intestinal mucosa
activates trypsinogen into trypsin.
Trypsinogen
Trypsin + Inactive peptide
The activated trypsin then activates the other enzymes of pancreatic juice.
Chymotrypsinogen is a proteolytic enzyme that breaks down proteins into peptides.
Chymotrypsinogen
Proteins
Chymotrypsin
Peptides
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Class XI
Chapter 16 – Digestion and Absorption
Biology
Carboxypeptidases act on the carboxyl end of the peptide chain and help in releasing
the last amino acids.
Peptides
Smaller peptide chain + Amino acids
Action of bile juice
Bile juice has bile salts such as bilirubin and biliverdin which break down large, fat
globules into smaller globules so that pancreatic enzymes can easily act on them.
This process is known as emulsification of fats. Bile juice also makes the medium
alkaline and activates lipase. Lipase then breaks down fats into diglycerides and
monoglycerides.
Action of intestinal juice
Intestinal
juice
contains
a
variety
of
enzymes.
Pancreatic
amylase
digests
polysaccharides into disaccharides. Disaccharidases such as maltase, lactase,
sucrase, etc., further digest the disaccharides.
The proteases hydrolyse peptides into dipeptides and finally into amino acids.
Dipeptides
Amino acids
Pancreatic lipase breaks down fats into diglycerides and monoglycerides.
The nucleases break down nucleic acids into nucleotides and nucleosides.
Question 13:
Explain the term thecodont and diphyodont.
Answer
Thecodont is a type of dentition in which the teeth are
embedded in the deep sockets of the jaw bone. Ankylosis is
absent and the roots are cylindrical.
Examples include living crocodilians and mammals.
Diphyodont is a type of dentition in which two successive sets
of teeth are developed during the lifetime of the organism. The
first set of teeth is deciduous and the other set is permanent.
The deciduous set of teeth is replaced by the permanent adult teeth.
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Class XI
Chapter 16 – Digestion and Absorption
Biology
This type of dentition can be seen in humans.
Question 14:
Name different types of teeth and their number in an adult human.
Answer
There are four different types of teeth in an adult human. They are as follows:
(i) Incisors
The eight teeth in the front are incisors. There are four incisors each in the upper jaw
and the lower jaw. They are meant for cutting.
(ii) Canines
The pointy teeth on either side of the incisors are canines. They are four in number,
two each placed in the upper jaw and the lower jaw. They are meant for tearing.
(iii) Premolars
They are present next to the canines. They are eight in number, four each placed in
the upper jaw and the lower jaw. They are meant for grinding.
(iv) Molars
They are present at the end of the jaw, next to the premolars. There are twelve
molars, six each placed in the upper jaw and the lower jaw.
Hence, the dental formula in humans is
This means each half of the upper jaw and the lower jaw has 2 incisors, 1 canine, 2
premolars, and 3 molars. Hence, an adult human has 32 permanent teeth.
Question 15:
What are the functions of liver?
Answer
Liver is the largest and heaviest internal organ of the body. It is not directly involved
in digestion, but secretes digestive juices. It secretes bile which plays a major role in
the emulsification of fats.
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